Sodium fluosilicate, Na₂SiF₆, is a white crystalline powder much less soluble than sodium fluoride in water. It has been used as a dust and spray in the control of some insects on field crops, as a poison in cutworm, mole cricket, and grasshopper baits and is effective as a mothproofing agent for woolen fabrics. A large number of fluorine compounds, both inorganic and organic, have been patented for use as mothproofing agents.

Some compounds of mercury are used as insecticides. Mercuric chloride (corrosive sublimate), HgCl₂, and mercurous chloride (calomel), HgCl, are used against fungus gnats, earthworms, cabbage maggots, and onion maggots. Mercuric chloride is also used for the treatment of dormant gladiolus corms and as a fungicide and germicide. Formulations containing mercury compounds are sometimes used to control insects affecting man and animals.

Pastes containing elemental phosphorus are made by grinding yellow phosphorus in the presence of water and then mixing with flour. Glycerin is sometimes used as an ingredient. Such pastes are effective against the American cockroach.

Selenium compounds have been tested as insecticides, but because of their toxicity to man their use is not recommended on crops intended for human or animal consumption.

Sodium selenate, Na₂SeO₄, is a water-soluble salt. Plants can take it up from the soil in sufficient amounts to kill aphids feeding on the plants. A Product containing selenium and sulfur of the formula (KNH₄S)₅Se has been used in the Pacific Northwest to combat mites on apples and grapes.

The use of elemental sulfur and alkaline sulfides as insecticides and fungicides on field crops and in greenhouses dates back many years. The materials are elemental sulfur, sulfides, polysulfides or salts of some of the oxygen acids of sulfur. Elemental sulfur is used alone as a dust or in combination with other insecticides with many of which it is compatible. The sulfur is reduced to a very fine state of subdivision by grinding, precipitation, or sublimation.

Dusting sulfur, or conditioned sulfur, is finely divided elemental sulfur made into a free-flowing powder by the admixture of 1 to 5 percent of clay, talc, gypsum, tri-calcium phosphate, or similar materials. Flotation sulfur, colloidal sulfur, and precipitated sulfur refer to finely divided sulfur formed as a result of chemical reactions of sulfur-containing compounds with other compounds. Wettable sulfur is finely divided sulfur that has been treated with wetting agents of various kinds to render it wettable by water and thus susceptible to suspension in spray formulations. The alkaline sulfides and polysulfides, sometimes referred to as soluble sulfurs, are prepared by the reduction of the salts of some of the oxygen acids of sulfur or by the action of alkaline solutions on elemental sulfur. The most important compounds of this class are the polysulfides of calcium, ammonium, barium, and sodium.

Calcium monosulfide, CaS, has been used to a limited extent. It is formed by the reduction of calcium sulfate. Liquid lime-sulfur or calcium poly-sulfide, CaS,,, is formed by the reactions between calcium hydroxide and elemental sulfur when they are boiled together in water. It is assumed to contain a mixture of the sulfides up to and including the pentasulfide, CaS₅. The theoretical reaction between 3 moles of hydrated lime, Ca(OH)₂, and 12 moles of sulfur results in the formation of 2 moles of calcium pentasulfide, CaS₅, 1 mole of calcium thiosulfate, CaS₂O₃, and 3 moles of water, H₂O.

Dry lime-sulfur is made by adding a stabilizer such as cane sugar to liquid lime-sulfur and evaporating to dryness. Self-boiled lime-sulfur is made by utilizing the heat of hydration or slaking of quicklime, CaO, to carry on the reactions with sulfur.

Ammonium polysulfide and sodium polysulfide are made by passing hydrogen sulfide gas, H₂S, into ammonium or sodium hydroxide containing excess sulfur. It is supposed that the chemical reactions are similar to those taking place in the preparation of lime-sulfur.

Sulfur is used under some conditions for the control of potato leafhopper, the cotton fleahopper, tomato psyllid, mites, and plant bugs.

Organic sulfur compounds, including thiocyanates, xanthates, and thiurarn disulfides, have some insecticidal properties although they are used largely as fungicides.

Sulfur dioxide, SO₂, made by burning sulfur, is sometimes used to kill insects in closed spaces.

Thallium sulfate, TI₂SO₄, sometimes is used as the toxic agent in ant poisons.

Several zinc compounds are in limited use as insecticides. Zinc sulfate, ZnSO₄, is sometimes used in place of copper sulfate in reactions with hydrated lime to form a zinc bordeaux mixture that has special uses. Zinc chloride, ZnCl₂, is used to protect against termites.

R. H. CARTER is a chemist in the Bureau of Entomology and Plant Quarantine, assigned to the division of insecticide investigations at the Agricultural Research Center at Beltsville, Md. After graduation from Morning-side College and the State University of Iowa, he was employed in chemical research in the Chemical Warfare Service for 10 years. Since joining the Department of Agriculture in 1927, he has been engaged in research in the development of insecticides, investigations of spray residue problems, and toxicological investigations of the effects of insecticide materials on farm animals.